1. Field of the Invention
Integrated circuits include various components that can be destroyed by the occurrence of overvoltages. A particular problem is the protection against electrostatic overvoltages of the terminals of integrated circuits during their final manufacturing steps or handling before mounting. These electrostatic overvoltages are, for example, caused when two pads or pins of an integrated circuit are contacted by a handling tool or the fingers of a user.
2. Discussion of the Related Art
It should be noted that protection circuits against electrostatic overvoltages must be active even if the integrated circuit is not connected and, more particularly, even if its ground terminals are not interconnected. In addition, these protection circuits against electrostatic pulses must not impair the operation of the integrated circuit once it is connected.
FIG. 1A shows a conventional protection scheme of an integrated circuit against electrostatic pulses including a single power supply source connected between a high voltage pad VVD.sub.1 and a low voltage pad VSS.sub.1. Each input/output pad p1-pn of the circuit is connected to a supply bus R1 connected to the high voltage pad VDD.sub.1 through an insulating diode D1 having its anode connected to the pad and its cathode connected to the supply bus R1. Each pad P1-Pn is also connected to a supply bus R2 connected to pad VSS.sub.1 through an insulating diode D2 having its cathode connected to the pad and its anode connected to the supply bus R2. The supply bus R1 is connected to the bus R2 through a clipping device symbolized in the form of an avalanche diode Z1 having its cathode connected to the bus R1 and its anode connected to the bus R2. Any conventional unidirectional clipping system can be used as a clipping device Z1. The clipping device Z1 has an avalanche threshold voltage higher than VDD.sub.1 - VSS.sub.1 but, of course, lower than the value of the overvoltage against which the components of the integrated circuit are to be protected.
In practice, in integrated circuits, diodes D1, D2 and the clipping device Z1 are disposed inside the integrated circuit and buses R1 and R2 are metallizations formed on this integrated circuit. It should be noted that the surface area off the components corresponding to diodes D1 and D2 is relatively small, in contrast, to a relatively large surface area of the clipping device Z1 of the integrated circuit.
The circuit of FIG. 1A meets the requisites for protection against electrostatic pulses when the integrated circuit has a single external power supply source.
However, a drawback of the circuit of FIG. 1A is that, since each pad is connected to the high voltage supply through a forward biased diode and to the low voltage supply through a reverse biased diode, each pad cannot exceed 0.6 volt above the high voltage VDD or decrease down to -0.6 volt below the low voltage VSS (usually ground). In practice, in some cases, it is desirable that the pad voltage can exceed the highest supply voltage. For example, when an integrated circuit is in operation, if the high supply voltage is interrupted and grounded and if one of the pads is associated with a storing capacitor, this capacitor will be discharged through one of diodes D1 toward the power supply pad VDD.sub.1. Also, output amplifiers can be connected to inductive loads. When overvoltages occur, they are clipped by the connection toward the high power supply terminal, which may be undesirable.
Existing integrated circuits often include a plurality of power supply sources (up to 7 in some circuits). Each of these power supply sources includes a high power supply terminal VDD.sub.i and a low power supply terminal VSS.sub.i.
Conventionally, to protect a circuit having a plurality of power supply sources, the circuit of FIG. 1A is simply enlarged by duplicating it by circuits such as the circuit of FIG. 1B. Pads P1-Pn, that are part of circuit portions connected to a first power supply source VDD.sub.1 -VSS.sub.1, are connected by the circuit of FIG. 1A. Pads Pn+1-Pn+k, that are part of circuit portions connected to a second power supply source VDD.sub.2 -VSS.sub.2, are connected by a circuit analogous to the circuit of FIG. 1A, illustrated in FIG. 1B. This type of circuit is duplicated for each power supply source.
The use of this type of integrated circuit protection including a plurality of power supply sources has several major drawbacks.
A first drawback is that such a structure requires as many clipping devices Zi as power supply sources. However, as indicated above, these clipping devices occupy a relatively large integrated circuit surface area, which conflicts with the integrated circuit designer's desire to decrease the surface area of the integrated circuit.
A second, still more important, drawback of the simultaneous use of circuits such as those of FIGS. 1A and 1B in a same integrated circuit is that this type of protection is ineffective when an electrostatic pulse is generated between a pad P1-Pn associated with a first power supply source and a pad P.sub.n+1 -P.sub.n+k associated with a second power supply source. Then, current no longer flows through a clipping device to short the overload, which then propagates inside the integrated circuit and can be destroying.